Atec Chroma-19050-19070 Series - Manual (Page 2)

TECHNICAL NOTE
FLASHOVER DETECTION
OPEN/SHORT CHECK (OSC)
O.S.C function is used to check the connection is open or short circuit between instrument and DUT(equipment under test) before the Electrical Safety Test.
If the connection is bad between the instrument and DUT, sometimes like leads or relay oxidation, the judgment is also PASS. In some cases, the DUT is
short before testing. Testing continually leads to our instrument broken because suffered the high load current. Therefore, we have to check the open and
short circuit to ensure the test effectively and protect instruments.
Generally, the DUT have capacitive load (Cx) from tens to thousands of pF. If the connection opening, a capacitance will appear and then total capacitive
load is lower than that in normal condition. If the DUT shorting, total capacitive load is higher than that in normal condition. Therefore, we can measure the
value of capactive load to check the contact is good or not.
V
µA
Hi
Lo
If Circuit Shorted :
C >> C
x
Ground Fault Interrupt (GFI)
The requirement of test environment indicates that test equipment
is equipped with auto interrupt device so that Chroma develops into
Ground Fault Interrupt (GFI) function. When the current meter A
1
and A
2
detect the difference (i
2
-i
1
=i
H
) between the value i
1
and actual
i
2
test current over high, this device can cut the power transiently for
protecting human body safety. It is not only compliance with the safety
standard but also more safeguards for test personnel.
V
s
V
µA
Hi
Lo
C
x
Normal Condition
V
µA
Hi
Lo
C
c
If Circuit Opened :
C = C
c
*C
x
/(C
c
+C
x
) << C
x
Fast transient in Voltage or Current occured while Hi-Pot testing is called Electrical Flashover. Normally, in AC line frequency (50Hz/60Hz) or DC Hi-Pot
testing, the leakage current is the same as 50Hz/60Hz or DC (charge current is excepted). As shown in Figure leakage current varies smoothly.
On the other hand, electrical discharge occurred because of poor insulation in material, electrode gap or surface clearance etc., fast transient in leakage
current apparent as shown in figure. This is phenomenon of poor withstanding. Most of Electrical Safety regulations mention the necessity in Withstand
Strength Test. Nevertheless, general Hi-Pot tester detects the RMS value of leakage current only without capability to detect Flashover. Therefore,
FLASHOVER detection function equipped with Hi-Pot tester is necessary.
10
-10
Test Voltage Waveform
Gap Discharge Voltage
180 360
Leakage Current (normal capacitive)
Figure 1 : Normal Leakage Current Waveform
0
Figure 2 : Leakage Current Waveform when flahover occurred
10
-10
Test Voltage Waveform
Gap Discharge Voltage
180 360
Leakage Current (abnornmal with flashover)
0
Hipot Tester
A2 A1
Return
DUT
HV
i
2
i
1
i
H
Operator
V
s
C
x
V
s